This　study　addresses　the　challenges　of　low　diffusion　efficiency　and　minimal　coercivity　enhancement　in　Ce-based　sintered　magnets　through　the　grain　boundary　diffusion　(GBD)　technique.　By　conducting　a　comprehensive　experimental　and　theoretical　analysis　of　La-substituted　Ce-based　magnets,　we　have　uncovered　the　detrimental　impact　of　the　REFe2　phase　on　grain　boundary　(GB)　structure　and　the　diffusion　process.　Our　findings　confirm　that　an　optimal　La　substitution　for　Ce　effectively　mitigates　these　issues.　This　substitution　reduces　the　stability　and　Dy　content　of　the　REFe2　phase　while　enhances　the　continuity　of　the　lamellar　GB　phases.　Consequently,　the　Dy　element　utilized　in　the　diffusion　process　achieves　a　greater　depth　of　penetration　and　a　more　uniform　distribution,　leading　to　a　significant　coercivity　increase　of　4.28　kOe　in　the　GBD　magnet　with　the　ideal　La　content,　an　8.35　%　improvement　over　the　La-free　magnet.　This　research　reveals　the　underlying　mechanisms　and　offers　a　viable　solution　for　enhancing　the　GBD　effect　in　Ce-based　sintered　magnets.　Furthermore,　it　underscores　the　pivotal　interplay　between　phase　stability,　grain　boundary　structure,　and　diffusion　dynamics　in　the　advancement　of　nextgeneration　GBD　techniques　for　permanent　magnets.